Date: Wed, 2 Oct 2002 07:59:48 -0400
Reply-To: "G.M.Bulley" <gmbulley@BULLEY-HEWLETT.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "G.M.Bulley" <gmbulley@BULLEY-HEWLETT.COM>
Organization: Bulley-Hewlett
Subject: Re: On the Vanagon Cooling System
In-Reply-To: <74.23f80a7b.2acbe4c3@aol.com>
Content-Type: text/plain; charset="us-ascii"
FG--"Ok, so what is the capacity of the Vanagon radiator core. Answer
(thank you, archives), 23.250 (W) x 16.50 (H) x 1.75 (T) for 671.34
cubic inches. I therefore have effectively twice the cooling capacity in
the radiator than I need."
Er, wait a minute. Does the volume figure for a radiator come from it's
exterior dimensions, fins and all, or from its true volume, (i.e.: fill
it with water, drain the water into a bucket, measure what's in the
bucket)?
Bringing your ideas to life,
G. Matthew Bulley
Bulley-Hewlett
Business: www.bulley-hewlett.com
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf
Of Frank Grunthaner
Sent: Wednesday, October 02, 2002 1:57 AM
To: vanagon@GERRY.VANAGON.COM
Subject: On the Vanagon Cooling System
Among those threads that triggered a negative response on my part were
several that addressed the apparently accepted wisdom of the marginal
capacity of the vanagon cooling system. Now some time ago, I put into
the
archives a summary of my measurements of the thermal performance of the
stock
I4 system and stated that it was actually strongly overcooled. Then
again,
some individuals return with the idea that removing the thermostat or
switching to a lower temperature one will give the system the necessary
extra
capacity it was lacking. Well, while licking my wounds and sitting here
amazed at what beta blockers can do to your general energy level, I
decided
not to complete my physicians recommended text of the day ("35 Delicious
Ways
to Prepare a High-Fiber Diet of Sawdust") and returned to some of my
cooling
system notes. Unfortunately for you, I decided to share them with list
before
Tom blocks me for needlessly verbose posting.
To begin, there are several key issues: 1) the thermal handling
capability of
the radiator, 2) the flow rate of the water pump, 3) the flow
resistance of
the plumbing going fore and aft and 4) the level of energy generated by
the
hot air pump in the rear whether it be a lowly I4, a perverse WBXer or
an
exhaulted Subie 6.
1). Radiator capacity. There are well known engineering rules for
determining
the necessary effective thermal handling capacity of the closed
loop/water/ethylene glycol cooling system. The rules digest to a volume
ratio. The core volume of the radiator must be at least a multiple of
the
engine displacement. The baseline factor is a multiplier of 2.0 (that
is,
engine displacement of x, then radiator core volume of 2x). This factor
is
then adjusted by known inefficiencies and additional loads. The inline
engine
adds 0.1 to the ratio; outside temperatures above 105 F, add 0.2; for a
small
engine in a large vehicle, add 0.2; for air conditioning add 0.3; for a
small
tight engine compartment, add 0.3; for a standard transmission subtract
0.1;
for a full fan shroud, subtract 0.2; for a horizontal flow radiator,
subtract
0.2; for a diesel engine add 0.6; for medium trailer towing, add 0.2. It
goes
on and on. For my interesting case (inline, hot, small, AC, tight,
manual,
shroud and crossflow with tow) I come up with a factor of 2.8 times
displacement. For my application, the 2.0L Audi 3A motor displaces 121
cubic
inches, so I need a radiator core volume of 338 cubic inches. If I had a
2.0L
TDi with an automatic, my ratio would be 3.5 and my core requirement
would be
423 cubic inches. The Subaru guys don't like these long winded posts so
they
will have to do the math.
Ok, so what is the capacity of the Vanagon radiator core. Answer (thank
you,
archives), 23.250 (W) x 16.50 (H) x 1.75 (T) for 671.34 cubic inches. I
therefore have effectively twice the cooling capacity in the radiator
than I
need. Overcooled says I. Good design says I. Of course, if its broken
fix it,
but no need to improve the engineering. Along this line, I should point
out
that the factory Ford Mustang with HiPo 302 cid V8 engine ships with 648
cubic inch radiator core and this calculation suggests that it needs 698
cubic inches. Good VW design. This discussion actually assumes Al core
radiators and Cu would be somewhat better.
2). Flow rate of the water pump. Well, this has proven hard to lock
down. VW
holds that this info is classified and only available in Slovenian!
Several
specs are given for industrial engines of 1.0 to 1.4L displacement.
These are
given as 15 gallons per minute at 2000 rpm. Several V8 engine design
manuals
call for 5 gallons per minute at idle and 22 gpm at 4000 rpm. Well
instrumented tests of GM 4 cylinder engine water pumps give the same
flow
rates as each side of the Chevy small block pump: 14.37 GPM at 2000 rpm
pump
shaft speed and 37.08 GPM at shaft speeds of 5000. All these numbers are
without cavitation. I therefore assume that the VW water pump driven at
proper shaft speeds is generating a flow rate of about 5 GPM at idle and
better than 35 at 6000 rpm. This corresponds to 20 L/Min at idle and 150
L/Min at 6K. That's a complete coolant capacity exchange every 50
seconds at
idle and every 6.5 seconds at 6000 rpm. More than adequate by any
reference.
3). Plumbing flow resistance. Well, for this exercise I did some
measurements
on one of my Saturday Morning Junkyard Constitutionals. The entrance and
exit
radiator ports on the VW Fox (7 examples) are 25.0 mm in diameter. These
ports on the Audi 5000T (big turbocharged beastie, 2.4 L I think) are
27.5 mm
in diameter and the inlet/outlet tubes on the diesel Vanagon I found
('82)
were 43 mm in diameter. Now the resistance to flow of an incompressible
fluid
through a pipe is directly proportional to pipe length and inversely
proportional to the internal pipe diameter. Taking into account these
relationships and assuming a to and fro length of 135 inches for the
Vanagon
pipes, we can compare the Vanagon plumbing resistance to that of the
Audi.
Such a calculation says the Vanagon tubes have a flow resistance of the
equivalent of 45 inches of Audi plumbing. Now the actual Audi run is
closer
to 24 inches. But the Vanagon tubes have a lower roughness and
frictional
loss (metal or hard plastic vs. rubber). The Audi hoses I saw were
corrugated
(more losses) and had several complex bends. In the end the best way to
measure reality is to do a pressure vs. flow plot before bolting up the
conversion engine (to paraphrase the elderly retired over the hill
Suburu
operating gent). Measurements rule! Nonetheless, I would argue that the
VW
engineers sized the Vanagon plumbing for similar flows (within 20%) of
that
obtained in the longitudinal and transverse cars in their stable.
Certainly
fully compatible with radiator size and pump capacity. Good design I
say.
Certainly compatible with anything the Subie or TIICO or Turbocharged
crowd
can throw at it.
Of course, this neglects head pressure when the radiator is 45 degrees
above
the engine and the driver is holding on with white knuckles, too
frightened
to look at the coolant gauge. This also neglects the direct injection of
combustion gases into the cooling jacket.
4). Energy injected. Well, this is actually treated by the general
capacity
discussion of part 1. But for reference, consider that the power
generated by
the hot air engine is roughly one third of the power processed. One
third
goes out the exhaust as heat and about one third goes into the cooling
system
for dissipation. So if your conversion pumps out a real level of 30%
more
horses than the WBxer, then 30% more heat has to be processed by the
cooling
system. Lets see, Diesel ... 42 Hp, Turbo Audi about 160 Hp! Glad we
have
that extra capacity.
In closing, I want to take a swipe at the thermostat wisdom. It is true
that
removing the thermostat leads to block overheating. It is not true that
this
is due to higher flow through the radiator - going too fast for cooling.
The
reason is that a minimum head pressure must be maintained across the
water
pump to eliminate cavitation. Lower temperature thermostats just throw
your
money down the drain. Lower engine efficiency. The engine operates more
and
more efficiently as the coolant temperature goes up to 105 C (for the
ethylene glycol/water system). The peak efficiency is around 122 C with
pure
propylene glycol. Of course, at 87 C, the hot spots in the block are
probably
well above 170 C! Small differences in mean control temperature are
irrelevant to the thermal stability of the system ... as I just tried to
show.
With apologies for length, I'm glad I finally got that off my chest!
Frank Grunthaner
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